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The term “Event” is really overloaded. There are many different utilities that leverage events. Event Sourcing, Event Carried State Transfer, and Event Notifications. None of these are for the same purpose. When talking about an Event Based architecture, realize which one you’re using and for what purpose.
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With the popularity of Microservices, Event Driven Architecture, Event Sourcing, and tooling, the term “Events” has become pretty overloaded and I find has been causing some confusion.
There’s a lot of content available online through blogs or videos that are often using the term Event incorrectly or in an ambiguous way. Almost every time I see the term event being used incorrectly is when the topic being covered is Microservices or Event Driven Architecture.
Here are the three different ways that the term “Event” is being used and in what context or pattern and for what purpose.
Event Sourcing is a different approach to storing data. Instead of storing the current state, you’re instead going to be storing events. Events represent the state transitions of things that have occurred in your system. Events are facts.
To illustrate the exact same product of SKU ABC123 that had a current state quantity of 59, this is how we would be storing this data using event sourcing.
This means that your events are the point of truth and you can derive current state from them.
The confusion when talking about Event Based Architecture comes in because I most often see people refer to Event Sourcing in a Microservices architecture as a means to communicate between services. Microservices and event sourcing are orthogonal. You do not need to be event sourcing in order to have a Microservice. Event sourcing has nothing to do with communication to other services. Again, event sourcing is about how you store data.
The reason, I believe, that there’s confusion is because when you’re event sourcing, you might be tempted to expose those events to other services. I say tempted because the events you’re storing as facts are not something you directly want to expose to other services. These events are internal and not used for integration.
Also, depending on the database or event store you’re using, it may also act as a message broker. This is often used within a service/boundary in order to create projections or read models from your event stream.
For more on projections, check out my post Projections in Event Sourcing: Build ANY model you want!
Event Carried State Transfer
The most common way I see events being used and explained is for state propagation. Meaning, you’re publishing events about state changes within a service, so other services (consumers) can keep a local cache copy of the data.
This is often referred to as Event Carried State Transfer.
The reason services will want a local cache copy of another service’s data, is so they do not need to make RPC calls to other services to get data. The issue with making the RPC call is if there are issues with availability or latency, the call might fail. In order to be available when other services are unavailable, they want the data they need locally.
In the example above, Warehouse and Billing require the Sales service. If the Sales service is unavailable, they may also be unavailable. To alleviate this, if they have the relevant data they need locally, it prevents them from having to make RPC calls to Sales.
What this looks like in practice is to use fat messages that generally contain all the data related to an entity.
Sales will publish a ProductChanged event that both the Warehouse and Billing will consume to update their local cache copies of a Product.
The contents of ProductChanged will generally look something like this:
The event will represent the entire current state of the entity. Meaning these events can get pretty large depending on the size of the entity.
While this approach is often used, I’d argue in most places if you’re doing this, you probably have some boundaries that are wrong. For more on defining boundaries check out my post on Defining Service Boundaries by Splitting Entities
Events as Notifications
The reason why Event Carried State Transfer is so popular when discussing Event Based Architecture is that Events will be used for notification purposes to other services, however being used incorrectly.
Most times events used for notifications are generally pretty slim. They don’t contain much data. If a consumer is handling an event but needs more information, to, for example, react and perform some action, it might have to make an RPC call back to the producing service to get more information. And this is what leads people to Event carried State Transfer, so they do not have to make these RPC calls.
To illustrate, the Sales service publishes an OrderPlaced event.
The Billing Service is consuming this event so it can then create an Invoice.
But because the event doesn’t contain much information, the Billing service then needs to make an RPC call back to Sales to get more data.
And this is how people then land on using Event carried State transfer to avoid this pattern.
Again, I sound like a broken record. But the likely reason this is occurring is because of incorrect boundaries. Services should own the data that relates to the capabilities they perform.
If you do have boundaries correct and each service has the relevant data for its capabilities, this means that events are used as a part of a workflow or long-running process. Events are used as notifications to tell other services that something has occurred.
To illustrate this again, Sales is publishing an OrderPlaced event that Billing is consuming.
Since Billing has all the data it needs, it creates an Invoice and publishes an OrderBilled event.
Next, the Warehouse service will consume the OrderBilled event so that it can create a ShippingLabel for the order to be shipped.
Once the shipping label has been created by the Warehouse, it publishes a LabelCreated event.
Finally, Sales is consuming the LabelCreated event so that it can update it’s Order status to show that the Order has been billed and is ready to be shipped.
This is called Event Choreography and is driven by events that are used for notifications. For more info check out my post Event Choreography & Orchestration (Sagas)
Event Based Architecture
Hopefully, this clears up some confusion about how the term even is used in different situations around event based architecture. I also hope it illustrates why Event Carried Transfer exists and the problem it’s trying to solve. However, that problem may likely be caused by incorrect boundaries.
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